form1.cs

private void bindingSource1_CurrentChanged(object sender, EventArgs e)
{
    // retrieve the current DataRow
    DataRow drCurrent = ((DataRowView)bindingSource1.Current).Row;

    // create a temporary copy with only the current row
    DataTable dtTemp  = drCurrent.Table.Clone();
    dtTemp.ImportRow(drCurrent);

    // merge the template with the cloned table
    mailMerge1.Merge(dtTemp);
}